Cleaning of nanostructures

ABSTRACT

The present invention relates to a method for removing a polymeric material from a surface of a nanostructure. The method includes applying, by a scanning probe microscope, an electrical field between a probe tip of the scanning probe microscope and the nanostructure, and simultaneously scanning over the surface of the nanostructure. Thereby, bonds connecting the polymeric material to the surface of the nanostructure are broken. A further step includes cleaning the surface of the nanostructure. A scanning probe microscope for performing such a method and a computer program product for controlling the scanning probe microscope are also disclosed.

BACKGROUND

The invention relates to a method for removing polymeric materials froma surface of a nanostructure. The invention further relates to acorresponding scanning probe microscope adapted to perform such a methodand a corresponding computer program product.

One major challenge for the scaling and commercialization ofnanostructures, in particular for 1-dimensional (1D) and 2-dimensional(2D) carbon nanostructures such as carbon nanotubes, graphene andgraphene nanoribbons, lies in the efficient post processing of thesematerials after lithographic processing. The lithographic processing maybe e.g. required to connect these materials between metal electrodes toarrange them e.g. in a transistor configuration.

Commonly used polymeric resist materials have a strong affinity andadhesion to carbon nanostructures. Common approaches use chemical orthermal treatment to remove the excessive polymeric materials, e.g.rapid thermal annealing or sample rinsing.

SUMMARY

According to a first aspect, the present invention is embodied as amethod for removing a polymeric material from a surface of ananostructure. The method comprises applying, by a scanning probemicroscope, an electrical field between a probe tip of the scanningprobe microscope and the nanostructure. The method further comprisesscanning over the surface of the nanostructure. Thereby bonds connectingthe polymeric material to the surface of the nanostructure are broken. Afurther step comprises cleaning the surface of the nanostructure.

According to a second aspect, the invention is embodied as a scanningprobe microscope. The scanning probe microscope comprises a probe tiphaving an apex tip area of more than 300 nm² and a voltage source forapplying a bias voltage between the probe tip and a nanostructure. Thescanning probe microscope further comprises a sample positionerconfigured to position the nanostructure in relation to the probe tipand a system controller configured to control the scanning probemicroscope. The scanning probe microscope comprises a cleaning mode thatis configured to apply an electrical field between the probe tip and thenanostructure and to scan over the surface of the nanostructure in orderto break bonds connecting a polymeric material to the surface of thenanostructure.

According to a further aspect, the invention can be embodied as acomputer program product for controlling a scanning probe microscopeaccording to the second aspect. The computer program product comprises acomputer readable storage medium having stored thereon programinstructions executable by the controller of the scanning probemicroscope to cause the scanning probe microscope to perform a cleaningmode. The cleaning mode comprises applying an electrical field betweenthe probe tip and the nanostructure and scanning over the surface of thenanostructure. Thereby bonds connecting a polymeric material to thesurface of the nanostructure are broken.

Devices and methods embodying the present invention will now bedescribed, by way of non-limiting examples, and in reference to theaccompanying drawings. Technical features depicted in the drawings arenot necessarily to scale. Also some parts may be depicted as being notin contact to ease the understanding of the drawings, whereas they mayvery well be meant to be in contact, in operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of a scanning probe microscope according toan embodiment of the invention;

FIG. 2a shows cross sectional view of a probe tip of a conventionalatomic force microscope;

FIG. 2b shows a cross-sectional view of probe tip according to anembodiment of the invention; and

FIG. 3 shows a flow chart of a method for removing polymeric materialsfrom a surface of a nanostructure according to an embodiment of theinvention.

DETAILED DESCRIPTION

In reference to FIGS. 1-3, general aspects of the invention andcorresponding terms are first described.

2-dimensional materials, also denoted as 2D materials or single layermaterials, may be defined as a class of materials, more particularly aclass of nanomaterials, defined by their property of being merely one ortwo atoms thick. One popular example of a 2-dimensional material isgraphene, a material constituted by a single layer of carbon atomsarranged in a hexagonal crystal lattice. 2-dimensional materials areconsidered to have many interesting applications in particular forfuture semiconductor technologies.

1-dimensional materials, also denoted as 1D materials, may be defined asnanostructures having a ratio of length to width greater than 1000.1-dimensional materials are also often referred to as nanowires. Thediameter of such nanowires is in the order of tens of a nanometer orless.

Embodiments of the invention provide techniques using a modifiedscanning probe tip in combination with a locally applied electric fieldunder a specified loading force to break adhesion between polymericmaterials and a surface of a nanostructure and to remove the polymericmaterials without inducing chemical or electrical alterations to ananostructure of interest.

FIG. 1 shows a scanning probe microscope 100 according to an embodimentof the invention. The scanning probe microscope 100 is embodied aselectrostatic force microscope and is accordingly adapted to apply anelectrical field to sample nanostructures of interest. The scanningprobe microscope 100 is configured to operate in a non-contact scanningmode, i.e. during operation there is no direct contact between thesample nanostructure and the probe tip.

The scanning probe microscope 100 comprises a probe tip 10 that isarranged on a cantilever 11. The probe tip has preferably an apex tiparea 12 of more than 300 nm². A preferred range of the apex tip area isbetween 300 nm² and 10000 nm². An even more preferred range is between1000 nm² and 7500 nm². The apex tip area 12 is preferably embodied witha flat surface. The apex tip area may be defined as the area that isarranged towards a surface 13 a of a nanostructure 13, also referred toas sample nanostructure 13. In other words, the apex tip area 12 is thearea of the probe tip 10 that is facing towards the surface 13 a of thesample nanostructure 13. The probe tip 10 is coated with a conductivematerial, e.g. a metal such as gold or platinum.

The scanning probe tip 100 further comprises a controllable voltagesource 14 for applying a bias voltage between the probe tip 10 and thesample nanostructure 13. The bias voltage is preferably a DC-biasvoltage. According to a preferred embodiment the bias voltage is in arange between 0.5 V and 2 V.

In addition, the scanning probe microscope 100 comprises a samplepositioner 15 configured to position the sample nanostructure 13 inrelation to the probe tip 10. The sample positioner 15 may position thesample nanostructure 13 in three directions x, y and z as indicated withthe arrows. Finally, the scanning probe microscope 100 comprises asystem controller 16 that is configured to control the scanning probemicroscope 100, e.g. the voltage source 14 and the sample positioner 15.

The scanning probe microscope 100 may comprise various scanning probemodes for performing probe scanning to analyze the surface ofnanostructures. These scanning probe modes may operate as known in theart. Furthermore, the scanning probe microscope 100 may be operated inone or more cleaning modes. In the one or more cleaning modes thescanning probe microscope 100 applies an electrical field between theprobe tip 10 and the nanostructure 13 in order to break bonds thatconnect a polymeric material 17 arranged on the surface 13 a of thenanostructure 13 to the nanostructure 13. The polymeric material 17 maybe e.g. a lithographic resist material that has remained afterlithographic processing of the nanostructure 13. In FIG. 1 the polymericmaterial 17 is represented by small rectangles in a simplified andschematic way to illustrate the principle of embodiments of theinvention. The bonds that connect the polymeric material 17 to thenanostructure 13 may be in particular based on adhesive forces.

In the cleaning mode the system controller 16 controls the scanningprobe microscope in such a way that a loading force is applied betweenthe probe tip 10 and the nanostructure 13. According to a preferredembodiment the loading force is in a range between 0.5 nN and 10 nN. Theloading force is based on the force constant of the cantilever 11 and adistance d between the probe tip 10 and the nanostructure 13. In thecleaning mode the scanning probe microscope 100 operates in non-contactmode and the probe tip 10 experiences an attractive force towards thenanostructure 13. The attractive force is in particular based on van derWaals forces.

The distance d between the probe tip 10 and the sample nanostructure ispreferably in a range between 25 nm and 100 nm.

It should be noted that in contrast to a normal AFM non-contact mode,the cantilever 11 is not oscillated during the cleaning mode.

Furthermore, the system controller 16 controls the strength of theelectrical field such that the electrical field between the probe tip 10and the nanostructure 13 is in a range between 10² V/m and 10⁶V/m. Theelectrical field can be controlled by the bias voltage of the voltagesource 14. The bias voltage of the voltage source 14 is preferablybetween 0.5 V and 2 V.

According to embodiments the nanostructure 13 may be a carbonnanostructure, e.g. a carbon nanotube, graphene or a graphenenanoribbon. The nanostructure 13 may be embodied as 2-dimensional or as1-dimensional material such as a nanowire. If embodied as 2-dimensionalmaterial, the 2-dimensional material may be in particular graphene,transition metal dichalcogenide (TMD), MoS2, WS2, WSe2 or BN.

According to an embodiment the scanning probe microscope 100 maycomprise a tuning fork (not shown) instead of the cantilever 11.

FIG. 2a shows cross sectional view of a probe tip 20 of a conventionalatomic force microscope. Accordingly, the probe tip 10 comprises a sharppeak 20 a.

FIG. 2b shows a cross-sectional view of probe tip 10 according to anembodiment of the invention. The probe tip 10 comprises an apex tip area12 that is significantly larger than the area of the peak 20 a of theprobe tip 20. The probe tip 10 may be fabricated from the probe tip 20by focused ion beam cutting. FIG. 2b also shows in a schematic way anelectrical field E applied between the probe tip 10 and thenanostructure 13.

FIG. 3 shows a flow chart of a method 300 for removing polymericmaterials from a surface of a nanostructure according to an embodimentof the invention. Parts of the method may be performed by the scanningprobe microscope 100 of FIG. 1 and more particularly be the cleaningmode of the scanning probe microscope 100.

At a step 301, the scanning probe microscope 100 applies an electricalfield between the probe tip 10 and the nanostructure 13.

Simultaneously, at a step 302, the probe tip 10 is scanned over thesurface 13 a of the nanostructure 13 or over a region of interest of thesurface 13 a of the nanostructure 13. Thereby bonds connecting thepolymeric material 17 with the surface 13 a of the nanostructure 13 arebroken. Accordingly bonds connecting the polymeric material 17 to thesurface 13 a of the nanostructure 13 can be eliminated by methodsembodying the invention in a controlled manner. A part of the polymericmaterial 17 may be transferred during scanning probe operation to theprobe tip 10 and hence may stick to the probe tip 10.

At a step 303, the surface 13 a of the nanostructure 13 is cleaned.

Simultaneously and as a result of the cleaning, at a step 304, theremaining part of the polymeric material 17 whose bonds to the surface13 a have been broken in the previous steps is fully removed from thesurface 13 a.

The cleaning step 303 may be performed by treating the nanostructure 13in a cleaning liquid. The cleaning liquid has preferably a boiling pointof less than 80° C. to avoid that the cleaning liquid sticks to thesurface 13 a of the nanostructure 13. Preferably an ultrasonication maybe performed in the cleaning liquid. The cleaning liquid may be e.g.iso-propyl alcohol. Alternatively or in addition the cleaning step maybe performed by blow drying the surface 13 a of the nanostructure 13,e.g. with N₂ gas.

The method may be preferably performed with an apex tip area 12 of theprobe tip 10 in a range between 300 nm2 and 10000 nm2. The method may bepreferably performed with a loading force of the scanning probemicroscope between 0.5 nN and 10 nN. The strength of the electricalfield may be preferably in a range between 10² V/m and 10⁶ V/m.

According to embodiments multiple scans can be performed and the scanspeed can be adapted and varied to optimize the overall cleaningresults.

According to embodiments of the method the strength of the appliedelectrical field and the distance between the probe tip and thenanostructure is controlled in such a way that the bonds connecting thepolymeric material and the nanostructure are broken.

One advantage of the method embodying the invention is that it may beperformed in a scalable and efficient way. Furthermore, it may improvethe stability and performance of nanostructures. Methods embodying theinvention may effectively remove traces of polymeric materials/residueswithout changing the material properties such as electrical resistivityand tensile strength of the nanostructure 13.

The applied loading force also creates a local joule heating effect thatmay additionally serve to defragment the polymeric material without anyextra energetic penalty.

The metal coated probe tip may, due to its large apex tip area,efficiently serve to clean large areas of a surface of the nanostructureof interest. Thereby the need for multiple passes to clean thenanostructure may be reduced or avoided. Techniques according toembodiments of the invention may even be scaled further by employingmultiple probe tips on the cantilever.

As mentioned above, polymeric materials may be transferred duringscanning probe operation to the probe tip 10 and hence may stuck to theprobe tip 10. According to embodiments the probe tip 10 may be cleanedafter multiple runs to remove these polymeric materials from the probetip 10.

Aspects of the present invention may be a system, a method, and/or acomputer program product. The computer program product may include acomputer readable storage medium (or media) having computer readableprogram instructions thereon for causing a processor to carry outaspects of the present invention.

Aspects of the invention may be in particular embodied as a computerprogram product for controlling the scanning probe microscope 100 and inparticular to perform a cleaning mode of the scanning probe microscope100. The computer program product may be loaded e.g. into the systemcontroller 16 of the scanning probe microscope 100. The computer programproduct has a computer readable storage medium having stored thereonprogram instructions executable by the system controller 16 of thescanning probe microscope 100.

The computer readable storage medium can be a tangible device that canretain and store instructions for use by an instruction executiondevice. The computer readable storage medium may be, for example, but isnot limited to, an electronic storage device, a magnetic storage device,an optical storage device, an electromagnetic storage device, asemiconductor storage device, or any suitable combination of theforegoing. A non-exhaustive list of more specific examples of thecomputer readable storage medium includes the following: a portablecomputer diskette, a hard disk, a random access memory (RAM), aread-only memory (ROM), an erasable programmable read-only memory (EPROMor Flash memory), a static random access memory (SRAM), a portablecompact disc read-only memory (CD-ROM), a digital versatile disk (DVD),a memory stick, a floppy disk, a mechanically encoded device such aspunch-cards or raised structures in a groove having instructionsrecorded thereon, and any suitable combination of the foregoing. Acomputer readable storage medium, as used herein, is not to be construedas being transitory signals per se, such as radio waves or other freelypropagating electromagnetic waves, electromagnetic waves propagatingthrough a waveguide or other transmission media (e.g., light pulsespassing through a fiber-optic cable), or electrical signals transmittedthrough a wire.

Computer readable program instructions described herein can bedownloaded to respective computing/processing devices from a computerreadable storage medium or to an external computer or external storagedevice via a network, for example, the Internet, a local area network, awide area network and/or a wireless network. The network may comprisecopper transmission cables, optical transmission fibers, wirelesstransmission, routers, firewalls, switches, gateway computers and/oredge servers. A network adapter card or network interface in eachcomputing/processing device receives computer readable programinstructions from the network and forwards the computer readable programinstructions for storage in a computer readable storage medium withinthe respective computing/processing device.

Computer readable program instructions for carrying out operations ofthe present invention may be assembler instructions,instruction-set-architecture (ISA) instructions, machine instructions,machine dependent instructions, microcode, firmware instructions,state-setting data, or either source code or object code written in anycombination of one or more programming languages, including an objectoriented programming language such as Smalltalk, C++ or the like, andconventional procedural programming languages, such as the “C”programming language or similar programming languages. The computerreadable program instructions may execute entirely on the user'scomputer, partly on the user's computer, as a stand-alone softwarepackage, partly on the user's computer and partly on a remote computeror entirely on the remote computer or server. In the latter scenario,the remote computer may be connected to the user's computer through anytype of network, including a local area network (LAN) or a wide areanetwork (WAN), or the connection may be made to an external computer(for example, through the Internet using an Internet Service Provider).In some embodiments, electronic circuitry including, for example,programmable logic circuitry, field-programmable gate arrays (FPGA), orprogrammable logic arrays (PLA) may execute the computer readableprogram instructions by utilizing state information of the computerreadable program instructions to personalize the electronic circuitry,in order to perform aspects of the present invention.

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems), and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer readable program instructions.

These computer readable program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks. These computer readable program instructionsmay also be stored in a computer readable storage medium that can directa computer, a programmable data processing apparatus, and/or otherdevices to function in a particular manner, such that the computerreadable storage medium having instructions stored therein comprises anarticle of manufacture including instructions which implement aspects ofthe function/act specified in the flowchart and/or block diagram blockor blocks.

The computer readable program instructions may also be loaded onto acomputer, other programmable data processing apparatus, or other deviceto cause a series of operational steps to be performed on the computer,other programmable apparatus or other device to produce a computerimplemented process, such that the instructions which execute on thecomputer, other programmable apparatus, or other device implement thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

While the present invention has been described with reference to alimited number of embodiments, variants and the accompanying drawings,it will be understood by those skilled in the art that various changesmay be made and equivalents may be substituted without departing fromthe scope of the present invention. In particular, a feature(device-like or method-like) recited in a given embodiment, variant orshown in a drawing may be combined with or replace another feature inanother embodiment, variant or drawing, without departing from the scopeof the present invention. Various combinations of the features describedin respect of any of the above embodiments or variants may accordinglybe contemplated, that remain within the scope of the appended claims. Inaddition, many minor modifications may be made to adapt a particularsituation or material to the teachings of the present invention withoutdeparting from its scope. Therefore, it is intended that the presentinvention not be limited to the particular embodiments disclosed, butthat the present invention will include all embodiments falling withinthe scope of the appended claims. In addition, many other variants thanexplicitly touched above can be contemplated.

What is claimed is:
 1. A method for removing a polymeric material from asurface of a nanostructure, the method comprising: applying, by ascanning probe microscope, an electrical field between a probe tip ofthe scanning probe microscope and the nanostructure; scanning over thesurface of the nanostructure, thereby breaking bonds connecting thepolymeric material to the surface of the nanostructure; and cleaning thesurface of the nanostructure.
 2. A method according to 1, wherein anapex tip area of the probe tip is greater than 300 nm².
 3. A method asclaimed in claim 1, wherein the scanning probe microscope is embodied aselectrostatic force microscope.
 4. A method as claimed in claim 1,wherein a loading force of the scanning probe microscope is between 0.5nN and 10 nN.
 5. A method as claimed in claim 1, wherein the strength ofthe electrical field is in a range between 10² V/m and 10⁶V/m.
 6. Amethod as claimed in claim 1, wherein the polymeric material is alithographic resist material.
 7. A method as claimed in claim 1, whereinthe nanostructure comprises carbon.
 8. A method as claimed in claim 1,wherein the nanostructure is a 2-dimensional material.
 9. A method asclaimed in claim 8, wherein the 2-dimensional material is selected fromthe group of: graphene; transition metal dichalcogenide (TMD); MoS₂;WS₂; WSe₂ and BN.
 10. A method as claimed in claim 1, wherein thenanostructure is a 1-dimensional material.
 11. A method as claimed inclaim 1, wherein the cleaning step comprises treating the nanostructurein a cleaning liquid having a boiling point of less than 80° C.
 12. Amethod as claimed in claim 11, wherein the cleaning step comprisesultrasonication in the cleaning liquid.
 13. A method as claimed in claim1, wherein the cleaning step comprises blow drying the surface of thenanostructure.